Firefox can display bookmarks in a drop-down list (Alt+b), in a sidebar (Ctrl+b) or in a separate window (Ctrl+Shift+b). Is one of these the one that isn't displaying as expected?
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Edit: I forgot to mention the Bookmarks toolbar, which only has room for a few bookmarks, but is displayed all the time. You can turn it off and on using:
Alt+v (displays the classic View menu) > Toolbars > Bookmarks Toolbar<br>
right-click a blank area of the tab bar > Bookmarks Toolbar